The present invention relates to a catalyst for removing pollutants from exhaust gases, and more particularly to a catalyst to be used with internal combustion engines and gas turbines operated at above the stoichiometric ratio.
Exhaust gases from combustion processes in engines and gas turbines are one source of present day pollution of the environment. The air pollutants contained in exhaust gases, i.e. nitrogen oxides, carbon monoxide and a wide variety of unburnt hydrocarbons, are sufficiently known with regard to their effect in polluting the environment.
Primary methods of reducing emission of pollutants, e.g. recycling of exhaust gases or modification of combustion chambers, have not hitherto reduced the omission of pollutants to the required extent or have unacceptably reduced the efficiency of installations. Consequently secondary methods such as catalytic exhaust gas purification are also needed for internal combustion engines and gas turbines.
The exhaust gases produced by burning liquid or gaseous fuel in internal combustion engines and gas turbines, if operating above the stoichiometric ratio, cannot be purified by the three-way catalyst principle. Unburnt hydrocarbons and carbon monoxide in exhaust gas can be removed by catalytic oxidation over an oxidation catalyst, using the oxygen in the exhaust gas, to form carbon dioxide and water, which are compounds compatible with the environment. Nitrogen oxides, owing to the oxygen which they contain, can be removed only by methods of selective catalytic reduction. One well-tried reducing agent is ammonia, obtained if required from an ammonia-producing chemical such as urea, which readily reacts with nitrogen oxides on a suitable catalyst but only reacts to a slight extent with oxygen.
In prior art systems comprising internal combustion engines, the aforementioned exhaust gases are purified by the following process: In one conventional process, the exhaust gases, heated to between 400.degree. and 600.degree. C., flow directly from the engine to an oxidation catalyst, where carbon monoxide and hydrocarbons are oxidized to carbon monoxide and water by the oxygen in the exhaust gas. The exhaust gases are then conveyed over a heat exchanger and cooled to the temperature (350.degree. to 400.degree. C.) required for selected catalytic reduction.
After ammonia has been sprayed in and mixed with the exhaust gas, the nitrogen oxides react with the ammonia over a reduction catalyst in a downstream reactor, forming nitrogen and water. The amount of added ammonia depends on the desired conversion rate and the burden of nitrogen oxides in the exhaust gas. In practice, as a result of bunching, local excesses of ammonia continuously occur. This results in "ammonia drift", i.e. unreacted ammonia enters the stream of exhaust gas behind the reduction catalyst and can thus escape through the chimney into the atmosphere, constituting an undesirable secondary emission When sulfur-containing fuels (e.g. diesel fuel, heavy heating oil or biogases) are used, the ammonia drift is accompanied by reactions between ammonia and the sulfur dioxides in the exhaust gas, resulting in corrosive, sticky, efficiency-reducing deposits of ammonium hydrogen sulfate and/or ammonium sulfate in the heat-exchangers or other downstream parts of the plant. The downstream parts therefore have to be periodically washed, resulting in an additional waste-water problem.
The system has other disadvantages, i.e. the additional pressure loss resulting from the frequent widening and narrowing of the flow cross-section, and the additional high cost of the separate reactors.
German patent specification DE-PS 36 01 378 describes a "Method of purifying exhaust gas from combustion systems containing oxides of nitrogen and sulfur" but this cannot be directly applied to internal combustion engines. Production of sulfuric acid is uneconomic, since the sulfur dioxide concentration in the exhaust gases from internal combustion engines and gas turbines is too low. After being charged with the required amount of ammonia the exhaust gas heated to 250.degree. to 550.degree. C. is conveyed over two different kinds of catalyst disposed one behind the other in a reactor. In the first catalyst step, selective catalytic reduction of nitrogen oxides to nitrogen and water takes place. In the aforementioned method the downstream oxidation catalyst is designed for optimum sulfur trioxide production and resistance to acid and resistance to sulfur trioxide. In the case therefore of exhaust gases from internal combustion engines and gas turbines, which contain little sulfur dioxide, it is necessary to use a catalyst specially designed for catalytic oxidation of hydrocarbons and carbon monoxide.